Re: Wal sender process not moving past wait_event_type: IO and wait_event: WALRead

(I don't think this is a bug report..)

At Thu, 24 Feb 2022 19:51:41 +0530, Anurag Shrivastava <anurag(dot)shrivastava(at)hevodata(dot)com> wrote in
> Hi Postgres team,
> We are facing an issue where we are unable to read data from a logical
> replication slot after a certain period of time. Every time after dropping
> the slot, it works fine for a few days and then again we are not able to
> read from the slot(we have been unable to find any queries that might be
> causing this issue). Each time the walsender process is getting stuck
> wait event type: IO and wait_event: WALRead. We've tried this with
> pgoutput, test_decoding and wal2json, with all three we have faced
> same issue.
> Is there a way to be able to read the data from the same slot?

I guess you can see lines in server log like this.

could not read from log segment %s, offset %d: %m
could not read from log segment %s, offset %d: read %d of %d

If so, there's a possibility that you have a bad block in the pg_wal
partition and periodically step on that block. The slot drop causes
to skip the bad block to allow start replication, but WAL recycling
places the bad block in a future WAL segment file again and that
repeats.

(I'm not sure there's a case where write scceeds but read fails on the
same block.)

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center